EMG Nerve Conduction Studies 2

Question 1

Which pattern of NCS findings is most characteristic of a demyelinating polyneuropathy?

Accepted Answer

Prolonged latencies, slowed conduction velocities, and possible conduction block

Answer

Demyelinating neuropathies primarily affect the myelin sheath, causing markedly prolonged latencies, reduced conduction velocities (often below 70–75% of the lower limit of normal), and may show conduction block or temporal dispersion.

Question 2

At low-frequency repetitive nerve stimulation (2–3 Hz), a greater than 10% decremental response in CMAP amplitude is characteristic of which condition?

Accepted Answer

Myasthenia Gravis

Answer

Myasthenia Gravis shows a decremental CMAP response at low-frequency RNS due to progressive depletion of acetylcholine at the postsynaptic neuromuscular junction with repetitive stimulation.

Question 3

High-frequency repetitive nerve stimulation (20–50 Hz) produces a significant incremental (facilitated) CMAP response exceeding 100% in which condition?

Accepted Answer

Lambert-Eaton Myasthenic Syndrome

Answer

In Lambert-Eaton Myasthenic Syndrome, high-frequency RNS causes presynaptic calcium accumulation that triggers release of additional acetylcholine, producing the characteristic incremental CMAP response.

Question 4

When performing a median-to-ulnar sensory comparison study across the palm, a latency difference greater than how many milliseconds between the two nerves suggests carpal tunnel syndrome?

Accepted Answer

0.4 ms

Answer

A median-to-ulnar sensory latency difference greater than 0.4 ms across the same palm distance is considered abnormal and significantly increases sensitivity for detecting mild carpal tunnel syndrome.

Question 5

In axonal polyneuropathy, what is the expected NCS pattern?

Accepted Answer

Normal or near-normal NCV with reduced CMAP and SNAP amplitudes

Answer

Axonal neuropathies primarily destroy nerve axons, reducing CMAP and SNAP amplitudes while conduction velocity in surviving fibers remains relatively preserved.

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EMG Nerve Conduction Studies 2